(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming images using the toner based on electrophotography, such as an electrostatic copier, laser printer, facsimile machine or the like, as well as relating to an image forming apparatus using this device.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography is constructed so as to form an image by forming an electrostatic latent image on the surface of a photoreceptor drum, for example, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on the photoreceptor drum by development to a sheet of paper or the like, and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses supporting full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow simply as “developer”), which can present excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to produce suitably electrified toner.
The toner electrified in the developing device is supplied to the surface of a dual-component developer supporting member, e.g., a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Recently, the image forming apparatus of this kind is demanded to be made compact and operate at high speeds, and it is also necessary to electrify the developer quickly and sufficiently and convey the developer quickly and smoothly.
To deal with such demands, a developing device of a circulating mechanism has been adopted in the image forming apparatus in order to disperse added toner promptly into the developer and provide the toner with a suitable amount of charge. This circulating type developing device includes a developer conveying passages through which the developer is circulatively conveyed and auger screws (developer agitators) that agitate the developer while conveying the developer in the developer passages. In this arrangement, when the toner concentration in the developer becomes lower than a predetermined level, toner is added from the toner hopper to the developer conveying passage, and the added toner is agitated with the developer whilst being conveyed (see patent document 1: Japanese Patent Application Laid-open 2001-255723).
However, in the developing device that circulatively conveys the developer whilst agitating as shown in patent document 1, if the height of the top surface of the developer (which will be referred to hereinbelow as “the developer surface”) varies along the longitudinal direction of the developing device, the amount of the developer being drawn up by the developing roller (the amount of the developer supplied to the photoreceptor drum) fluctuates, causing the problem of image density unevenness along the longitudinal direction of the developing roller.
Further, when toner is newly added to the developer, the developer presents local variations in toner concentration and in the amount of static charge on the toner, which readily cause change in the volume density of the developer and variation in the developer surface. In particular, in the developing device of a circulative conveying type, since no conveying member such as an auger screw or the like is disposed at the communication path from one developer conveying passage to the other, the developer, if it is poor in fluidity, is prone to stagnate, triggering marked rise of the developer surface.